This invention relates to the manufacture of plastic net products and in particular to an improved plastic net product and method for manufacturing same.
The continuous extrusion of plastic net started in about 1956 with the process described in the Mercer U.S. Pat. No. 2,919,467. Since that time, many patents have issued in the United States as well as in other countries describing improvements and refinements in the continous extrusion process.
The initial extrusion process has developed along two basic lines: the first, in which plastic sheet is extruded and holes are formed therein to provide a net-like structure, and the second, in which individual plastic strands are extruded in an interconnecting network to provide the net-like structure. This invention is specifically concerned with a variation in the latter of the two.
Methods for practicing the latter technique are well known. For instance, U.S. Pat. Nos. 3,700,521; 3,767,353; 3,723,218; 4,123,491; 4,152,479 and 4,190,692 show apparatus and methods for making net by continuous extrusion of strands.
The disclosures of the above-mentioned issued patents are incorporated by reference into the present specification as are all of the patents which may be referred to hereinbelow in further description of this invention.
In all of these patents at least one set of strands is extruded through a plurality of spaced individual orifices. A second set of strands in the net structure may be extruded through a second set of spaced individual orifices or a second set of strands may be preferentially extruded periodically through a continuous annular orifice slit. In all cases the two sets of strands are extruded such that the individual strands intersect at an angle and form integral joints in the extruded plastic net. The resulting extruded flat sheet in one process or tube of plastic net in another process is cooled to set the plastic in the strands, as for example in a water bath, and the net is drawn away from the extrusion orifices by nip rolls or other suitable drawing means. When a tube of net is extruded, it is usually drawn over a cylindrical mandrel which may stretch the strands and enlarge the openings in the net structure. Such stretching of the strands over the mandrel preferentially orients the plastic but in practice the net is characterized as being "unoriented".
For many applications, it is desirable to further stretch the net strands and more fully orient the plastic and this may be done, where as in the case of a tube of extruded net, the tube is heated and stretched longitudinally to further elongate and orient the strands. Stretching the tube causes it to collapse while the tube is being stretched longitudinally. If the tube has been slit and formed into a flat sheet of extruded net, the flat sheet may be heated and one set of strands may be stretched and oriented in one direction, and in a second separate step, the second set of strands may be stretched to orient the strands in a second direction. Some plastic net may be oriented at room temperature but as a practical matter the net is heated to speed up and facilitate orientation of the net.
A significant problem in the manufacture of high temperature oriented netting such as nylon or other polyamides and polyester netting is that such materials have a low-melt-strength and degrade easily. The low-melt-strength of such polymer resins makes it difficult to produce a uniform extruded net and also makes it difficult to produce a quality oriented product.
Also, while the extrusion process works very well for the extrusion of non-polar polyolefins such as polypropylene, other resins which are polar such as the aforementioned nylon or other polyamides, and other low-melt-strength resins tend to stick to the extrusion die. The degradation of molten resin during extrusion is a problem because degraded material builds up on the die lips. This makes an equipment shut down necessary approximately every six-eight hours to clean the die parts and interrupts the continuous process.
Both low-melt-strength and resin degradation have been found to present major problems in the extrusion of net-like products of nylon, polyester and the like.
This invention has as an objective, the successful and continuous extrusion of low-melt-strength polymer net product without die build-up.
Also, since fusible net products are being used in greater and greater quantities as a reinforcing structure in fabric-to-fabric laminates such as paper toweling, reinforced tissue and the like, the net product of this invention may be used to improve these laminates. In some forms of the invention, the net product may also utilize a co-extruded outer layer of heat sealable polymer carried by a high temperature core as a fusible adhesive for heat bonding or laminating such tissues, fabrics and the like, together with the reinforcing net product sandwiched therebetween. For such uses, the extruded fusible net product of this invention has, among other advantages, that of uniformity, dimensional stability and high strength as compared to other reinforcing products.
It is, therefore, another object of this invention to provide an improved co-extruded net product which not only functions to reinforce such laminates but also provides the adhesive necessary to hold the laminate together.
These and other objects of the invention will be apparent from the description provided hereinbelow.